The present invention generally relates to molding processes and equipment for producing composite articles. More particularly, this invention relates to a molding process for producing perforated composite structures suitable for use in, as examples, nacelle and reverser acoustic panels of gas turbine engines.
A typical construction used in aircraft engine nacelle components (such as the engine inlet, thrust reverser cowls, and blocker doors) and engine duct flow surfaces is a sandwich-type layered structure comprising a core material between a pair of thinner sheets or skins, one of which is perforated. The core material is typically a lightweight honeycomb metallic or composite material. A variety of metallic and composite materials can also be used for the perforated (acoustic) skin, with common materials including aluminum alloys, fiberglass, and fabric materials (for example, a graphite fabric) impregnated with resin (for example, an epoxy resin). The perforations in the acoustic skin are the result of an acoustic treatment by which numerous small through-holes are formed to suppress noise by channeling pressure waves associated with sound into the open cells within the core, where the energy of the waves is dissipated through friction (conversion to heat), pressure losses, and cancellation by wave reflection.
A conventional process for producing perforated composite skins is to impregnate a graphite fabric with resin and then precure the impregnated skin. Pre-impregnated skins are bonded to opposite surfaces of a core material with adhesive under pressure and heat, typically performed in an autoclave, during which final curing occurs. Alternative conventional processes include co-curing where the skins are not pre-cured but are cured as part of the process of curing the adhesive to skin bond. Disadvantages associated with these processes include long cycle times, high capital investment, and difficulty when attempting to implement for complex geometries. Alternatives to the use of an autoclave include resin transfer molding (RTM) and vacuum-assisted resin transfer molding (VaRTM) processes.
For perforated skins used in some gas turbine engine applications, holes on the order of about 0.03 to about 0.06 inch (about 0.75 to about 1.5 mm) in diameter and hole-to-hole spacings of about 0.06 to about 0.12 inch (about 1.5 to about 3 mm) are typical, resulting in acoustic hole patterns containing seventy-five holes or more per square inch (about twelve holes or more per square centimeter) of treated surface. Given the large number of holes necessary to acoustically treat nacelle components and acoustic panels, rapid and economical methods for producing the holes are desirable.
Common processes currently employed to produce acoustic holes in acoustic skins include punching, mechanical drilling, and pin molding. Each of these processes has its limitations. For example, punching is typically practical for only relatively thin skins of one or two plies, and is often limited to producing fiberglass acoustic skins Mechanical drilling, which is often employed with graphite composite skins, typically drills one, two, or four holes at a time in a skin cured to its finished geometric shape. In addition to limited speed, mechanical drilling processes tend to be expensive due to the special tooling and machinery required to place the holes in the proper orientation on the contoured skin. Pin molding typically entails forcing a pre-impregnated composite skin material onto metallic or nonmetallic pin mats, after which the skin material undergoes an autoclave cure followed by removal of the pin mats. Such a process is slow and labor intensive with significant recurring costs arising from the need to replace worn pin mats. In addition, both mechanical drilling and forcing sharp pins through fibrous pre-impregnated materials result in breakage of fibers and a reduction of optimum laminate skin strength. None of these processes are well suited for perforating composite skins at relatively high rates while incurring minimal equipment, labor, and recurring costs.